Plasma processing apparatus and plasma processing method

ABSTRACT

A plasma processing apparatus having 90% or more of a side wall of an inner wall  101  of a reaction chamber  1  covered with a dielectric  102 , and equipped with an earthed conductive member  21   a  having an area of less than 10% of the side wall area of the inner wall  101  and having a structure to allow direct current from a plasma to flow therein, wherein the DC earth formed of the conductive member  21  is located at a position where floating potential of plasma (or plasma density) is higher than the floating potential of plasma  9  located near a wafer holding electrode  14  where there is relatively large wall chipping.

FIELD OF THE INVENTION

The present invention relates to a plasma processing apparatus andplasma processing method that reduce the damage to the inner walls ofthe reaction chamber that is caused by the application of highfrequencies to a substrate to be processed, and that enable stableprocessing to be performed for a long time.

DESCRIPTION OF THE RELATED ART

Along with the miniaturization and integration of semiconductor devicesand with the increase in the variety of component materials used in thedevices, the plasma processing apparatuses used for manufacturing thesemiconductor devices are required not only to be able to perform highlyaccurate processes but also to enable stable quantity output andenhanced cleanness. In a plasma etching apparatus, for example, highfrequency is applied to the wafer to be processed using reactive gasplasma, according to which the inner walls of the reaction chamber aredamaged by ions puttering due to high frequency power, and the innerwall material of the reaction chamber is chemically eroded by thereactive gas. Such sputtering or erosion of the wall material causes themetal contained in the wall material to enter the processed wafer andcause deterioration of the LSI circuit performance, and causes thechemical composition or the manner in which the high frequency ispropagated in the reaction chamber to vary gradually, thus making itimpossible to perform long-term stable processing. Further, if chemicalreaction occurs between the wall material and reaction products, causingcontaminants to deposit on the inner walls of the reaction chamber, thedeposits that are gradually grown during long term use may fall off fromthe walls and enter the circuit formed on the wafer as foreign matter,causing increase of percent defective of the products being processed.

In order to cope with this problem, in recent plasma processingapparatuses, the surface of components in the apparatus such as theinner walls of the reaction chamber and the substrate holder are coatedwith non-conductive (dielectric) material such as alumite (anodizedaluminum) that is stable to chemical reaction, or covered with quarts orpolymeric material.

However, if the region that generates the plasma is covered withdielectric, the flow of charged particles dispersing from the plasmabecomes unstable and the plasma potential is varied thereby, accordingto which stable processing becomes difficult. Further, the varyingpotential may cause damage to the wafers being processed. FIG. 6 showsthe measured results of the potential waveforms at the inner wallsurface of a plasma etching apparatus having a reaction chamber formedof aluminum with the whole inner wall surface being anodized, whereinthe measurement is performed using an oscilloscope having its sensormounted on the inner wall of the anodized aluminum reaction chamber.

On the other hand, as illustrated in FIG. 7 explaining the dielectriccoating portion formed on the inner wall surface of the reactionchamber, an alumina coating 102 as dielectric is equivalent to acondenser with a thickness d intervened between a plasma 9 and areaction chamber inner wall 101 with respect to high frequencies(frequency fHz). Thus, the high frequency can be propagated through thealumina coating and the inner wall functions as earth for the highfrequency even with the alumina film coating.

The processing conditions in this example corresponds to a typicaletching process, wherein CF₄ gas is used as reactive gas and plasma isdischarged with a pressure of 2 Pa, while a high frequency of 400 kHz isapplied by 140 W to a 12-inch Si wafer. As can be seen from FIG. 6, thepotential at the inner wall surface of the chamber is varied at 400 kHzin synchronism with the applied high frequency, and the potential isalso varied in a DC-like manner by the high frequency power (biaspower). This variation of potential at the inner wall surface is alsocaused according to various processing conditions, and especially, theDC-like potential variation has no reproducibility and shows unstablebehavior according to some discharge conditions. That is, the potentialat the surface of the inner wall 101 of the reaction chamber ismaintained at negative potential if the applied bias power is low, butis maintained at positive potential if the applied bias power becomeshigher. In the case shown in FIG. 6, the DC-like potential variation is,at maximum, over 20 V.

The relationship between the plasma potential near the inner wall of thereaction chamber and the damage to the walls caused by ion sputteringwill now be explained with reference to FIG. 8. Regarding the potentialof plasma at the front surface of the inner wall of the reactionchamber, a sheath is formed between the plasma and the wall, by whichpositive ions are accelerated toward the wall. The acceleration energyof ions depend on the plasma potential (ion acceleration potential) V1at the end of the sheath, wherein if the plasma potential V1 isincreased, the acceleration energy of ions is increased, and the amountof damage on the wall surface caused by the ions sputtering the wall isincreased. Therefore, if the plasma potential varies for approximately20 V in a DC-like manner as shown in FIG. 6, the voltage thataccelerates the ions from the plasma are increased by 20 V when the wallis sputtered, and as a result, the wall chipping speed is increased.

Further, if such potential variation of plasma occurs, an electric fieldis induced within the processed wafer surface, causing electrical damageto the circuit formed on the wafer and resulting in deterioration ofproduct property and increase of defective fraction.

In an inductively coupled plasma etching apparatus, there is noelectrode serving as a potential reference within the reaction chambersince an induction coil disposed outside the dielectric vacuum window isused to apply high frequency and generate plasma in the chamber, so theplasma potential tends to be varied, causing damage to the circuitformed on the wafer. Thus, an earth point serving as a potentialreference was disposed within the plasma processing chamber (refer forexample to Patent Document 1). It is disclosed in Patent Document 1 thatthe disposed earth point is formed of a conductive metal material andcan have a protective coating made for example of insulating ceramic.Thus, the disclosure does not consider the DC-like variation of plasmapotential, but only assumes the variation of plasma potential of highfrequencies that are able to pass through the insulative protectivecoating formed on the earth point surface. Further, as for the locationfor disposing the earth point, the disclosure mentions that it can belocated at a place in shadow with respect to the plasma generated withinthe plasma reaction chamber. According to this method, however, theDC-like potential variation is not necessarily stabilized, and if aninsulative protective coating is formed on the earth point surface, theDC-like potential variation as shown in FIG. 6 cannot be reduced. As forthe location for disposing the earth point, since it is not disposedwhere the plasma exists in high densities in the plasma reactionchamber, the effect of stabilizing the DC-like variation of the plasmapotential is small.

On the other hand, a plasma processing system is disclosed (for example,refer to Patent Document 2) that prevents the Variation of plasmapotential, comprising an earth electrode made of aluminum alloy as basematerial and having an alumite layer coating the base material that isdisposed around the circumference of the substrate holder, in which thealuminum alloy as base material is exposed in advance at prescribedpositions where the alumite coating tends to be damaged by the process,thereby preventing the variation of processing conditions by the alumitecoating coming off during the long term use. In such prior-art system,the area from which alumite is chipped away is predetermined, which isthe upper end of the earth electrode. The area at which the aluminumalloy base material is exposed corresponds to this area from which thealumite is chipped away, which is 2% of the process wafer area orsmaller. Therefore, this prior art disclosure only mentions the effectof suppressing the variation in processing conditions caused by thealumite being chipped away, and does not consider suppressing thepotential variation of plasma or reducing the damage of the earth.

In order to prevent the deterioration of properties of the formed LSIcircuit during the plasma process, it is very important to reduce theamount of metal impurities chipped away from the inner walls of thereaction chamber and entering the wafer. Therefore, it has becomeindispensable to cover the inner surface of the reaction chamber with aprotective coating made of chemically stable insulating materials.However, if the inner surface of the reaction chamber is covered withinsulating protective coating, the plasma becomes unstable, causingdamage to the LSI circuits or deteriorating the long-term stability ofthe process.

In order to solve such problems, an earth point was provided in theprocessing chamber as disclosed in above-mentioned Patent Document 1,but by simply providing an earth point, only the potential variation ofplasma was stabilized, and the problem of the reaction chamber beingdamaged by the ion sputtering caused by the high frequency applied tothe wafer could not be solved. That is, if the earth point is located atthe corner of the plasma reaction chamber, the plasma density cominginto contact therewith falls and the electric resistance between theplasma near the earth and the earth point becomes large, between whichoccurs the fall of potential, and the function as earth electrode isdeteriorated. Moreover, if a protective coating formed of an insulatingmaterial is provided on the earth point surface, the plasma potentialcannot be stabilized with respect to DC-like or low-frequency potentialvariations. Furthermore, the disclosure related to providing the earthpoint does not consider the effect of reducing the damage to the innerwall of the reaction chamber or to the earth caused by the highfrequencies applied to the wafer by providing the earth point.

On the other hand, the method disclosed in Patent Document 2 relates toremoving the alumite in advance at the end of the earth in the plasmareaction chamber that tend to be chipped away and effectively reducesthe variation of the process in long term, but it but does not reducethe damage to the earth. On the contrary, since the alumite serving as aprotective coating is removed from the end portion of the earth formedof aluminum alloy base material and disposed on the sidewall of theelectrode mounting the wafer, the aluminum alloy base material isdirectly exposed to plasma, by which the aluminum is directly subjectedto ion sputtering causing impurities to enter the wafer. Furthermore,the reactive gas used in the etching process comes into direct contactwith the base material, causing the material to be damaged by chemicalreaction and increasing the amount of metal impurities entering thewafer.

In general, the potential generated on the wall surface is distributedso that the potential decreases toward the wall surface as shown in FIG.8, by which the dispersion of ions is increased. The steady potentialdifference formed at this time that accelerate ions is theoreticallyseveral times the electron temperature (4.7 times in the case of argonplasma), which is approximately several V to over 10 V (refer toNon-Patent Document 1).

However, when high frequencies are applied to the wafer so as toirradiate ions on the wafer being processed, the potential at the innerwall surface varies by the high frequency. When the high frequency poweris increased, the potential variation intensity at the inner wallsurface is increased thereby, and the energy (V1) of the ions sputteringthe inner wall surface is enhanced, according to which the damage to thewalls is increased. For example, upon etching an insulating film, ionirradiation with relatively high energy must be performed to acceleratethe surface reaction, and a high frequency of approximately 200 W isapplied to a wafer with a diameter size of 300 mm. At this time, themaximum-minimum amplitude of potential at the wafer position is around1000 V, and based on this variation of wafer potential, the plasmapotential (V1) near the reaction chamber wall reaches as high as several10 V at maximum by the high frequency being applied. If the ions areaccelerated by this potential variation, the ions sputter the inner wallof the reaction chamber and causes aluminum and other metal elements ofthe wall material to be mixed into the plasma.

Patent Document 1:

Japanese Patent Laid-Open Application No. 2001-23967

Patent Document 2:

Japanese Patent Laid-Open Application No. 2001-267299

Non-Patent Document 1: “Principles of Plasma Discharges and MaterialsProcessing”, M. A. Lieberman, Translated by H. Sato, Published Nov. 20,2001 by ED Research Co., Page 116

SUMMARY OF THE INVENTION

The present invention aims at solving the problems of the prior artmentioned above. In other words, the object of the present invention isto provide a plasma processing apparatus and plasma processing methodcapable of reducing the amount of impurities in the reaction chamberwithout deteriorating the stability of plasma.

In order to solve the above-mentioned problems, the present inventioncovers 90% or more of the area of the inner wall of the reaction chamberwith dielectric, and provides on the inner wall of the reaction chambera DC earth formed of an earthed conductive member having an area of lessthan 10% on the inner wall and formed so that direct current flowstherein from the plasma. Further, the DC earth is positioned where thefloating potential of plasma (or plasma density) is higher than thefloating potential of plasma near the substrate holder (wafer holdingelectrode) where there is relatively intense wall chipping.

According to the present invention, the dielectric is a protectivecoating formed of insulating ceramic such as carbide, oxide or nitridelike SiC, boron carbide and alumite, and the thickness d of thedielectric coating is determined so that, with respect to therelationship between frequency f of the high frequency applied to thesubstrate and the dielectric constant ∈ of the dielectric, an impedanceper unit area R=d/(2πf∈) when the high frequency is propagated bycapacity coupling through the dielectric portion is 100 Ω or smaller.

According further to the present invention, a magnetic field generationmeans is disposed outside the reaction chamber to apply magnetic fieldto the plasma, and the DC earth is disposed at a position crossing amagnetic line of force that is closer to the substrate holder than amagnetic line of force that crosses the inner wall of the reactionchamber closest to the substrate.

According to the present invention, either a base material of the DCearth or a protective coating disposed on a surface of the DC earthcoming into contact with plasma is composed of conductive ceramic, SiC,Al or Al compound.

According to the present invention, when a base material of the DC earthis composed of a non-metallic material such as conductive ceramic, SiC,Al or Al compound, a conductive member having a conductivity of 1 Ωcm orless is disposed on the mounting surface of the DC earth by evaporation,spraying or interposing, thereby reducing the earth resistance of the DCearth.

According to the present structure, even if the inner wall is coatedwith a chemically stable protective coating so as to reduce the amountof metal impurities generated from the metal inner wall material of theplasma reaction chamber, the plasma near the inner wall is earthed inDC-like manner, so the plasma is prevented from becoming unstable, andthe LSI circuits will not be damaged. Further, as shown in FIG. 9, sincethe DC earth provides an earth at a position where the plasma potentialøp is high, the plasma potential at the inner wall portion and at theearth portion of the reaction chamber can be reduced. Thus, the voltageaccelerating the ions in the plasma can be cut down, and the wall damagecaused by ion sputtering can be suppressed.

That is, with respect to the electrons and ions which are chargedparticles that constitute the plasma, an electric field pulling back theelectrons that have smaller mass and tend to diffuse at high speed isself-generated in the plasma. As a result, a plasma potential (or plasmadensity) distribution as shown in FIG. 9 is formed in the plasma. Thepotential of the plasma is highest at the plasma generating portion, andis reduced gradually toward the inner wall of the reaction chambertoward which charged particles diffuse. The difference in potential ofthe plasma is varied by the difference in the arrangement of theapparatus, the plasma generating system or the discharge gas etc., butin a normal plasma, the energy of the electrons is several eV, and thepotential difference generated in the plasma is around several V toseveral tens of V. Moreover, in general, the plasma density is highestat the plasma generation area and is reduced toward the inner wall ofthe reaction chamber since plasma is diffused toward the wall. Thus, theposition for locating the DC earth where plasma potential is highcorresponds to where the plasma density is higher than near the innerwall of the reaction chamber.

Therefore, by disposing the DC earth at a location closer to the plasmagenerating region (where plasma density is highest) than the position ofthe earth portion (inner wall of the reaction chamber) or at the innerwall of the reaction chamber where wall sputtering must be suppressed,the plasma potential at the inner wall or at the earth portion wheresputtering becomes a problem can be reduced by δV.

By providing as the protective coating of the inner wall of the reactionchamber an insulating ceramic such as carbide, oxide or nitride likeSiC, boron carbide and alumite, and the thickness d of the dielectriccoating is determined so that, with respect to the relationship betweenfrequency f of the high frequency applied to the substrate and thedielectric constant ∈ of the dielectric, an impedance per unit areaR=d/(2πf∈) when the high frequency is propagated by capacity couplingthrough the dielectric portion is 100 Ω or smaller, the impedance R ofthe protective coating is maintained low with respect to highfrequencies, so the coating does not deteriorate the function of theearth to high frequencies. As a result, even when the inner wall of thereaction chamber is covered with protective coating, the impedance ofthe protective coating with respect to the high frequency currentflowing into the wall or the earth portion is increased, by which thevariation in the plasma potential can be suppressed, and the increase ofwall damage caused by ion sputtering can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the outline of a structure of aplasma processing apparatus according to the present invention;

FIG. 2 is an explanatory view showing the mounting position of aconductive member;

FIG. 3 is a graph showing surface potential waveforms measured at theinner wall of the reaction chamber equipped with a conductive memberaccording to the present invention;

FIG. 4 shows the relationship between the aluminum content of thedeposition in the reaction chamber and the area of the conductivemember;

FIG. 5 is an explanatory view showing how the conductive members ismounted;

FIG. 6 is a graph showing surface potential waveforms measured at theinner wall of the reaction chamber that is not equipped with theconductive member;

FIG. 7 is an explanatory view showing the function of a dielectricprotective coating disposed on the inner wall of the reaction chamber;

FIG. 8 is an explanatory view showing the relationship between theplasma potential and ion sputtering; and

FIG. 9 is an explanatory view showing the plasma potential distribution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

We will now describe in detail a plasma etching apparatus utilizingelectromagnetic waves in the UHF band, which is one example of theplasma processing apparatus according to a first preferred embodiment ofthe present invention with reference to FIG. 1. The plasma etchingapparatus is composed of a reaction chamber container 10 defining areaction chamber 1, a dielectric vacuum window 12, a gas discharge plate13 having gas discharge holes 131, a wafer holding electrode 14functioning as a substrate holder on which a wafer 4 is mounted, a fieldcoil 15, an exhaust outlet 16 for maintaining the inside of the reactionchamber container 10 in decompressed state, a plasma-generatinghigh-frequency electrode 17 to which high frequency is applied from aplasma-generating high-frequency power supply 18 for generating plasma 9inside the reaction chamber, a wafer biasing high-frequency power supply19 for supplying a biasing high-frequency power to the wafer holdingelectrode 14, and a conductive portion or member 21 a electricallyconnected to the reaction chamber container 10 and functioning as a DCearth. The surface portion of an inner wall 101 of the reaction chambercontainer 10 which is exposed to the plasma is coated with insulatingmaterial 102.

The reaction chamber 1 for plasma processing includes, at the upper areathereof, a dielectric vacuum window 12 for introducing the UHF-bandelectromagnetic waves for generating plasma 9, and a gas discharge plate13 made of dieletric material and having gas discharge holes 131 throughwhich reactive processing gas is introduced, and the exhaust from theetching process containing reaction products is evacuated through theexhaust outlet 16 disposed at the bottom area of the reaction chamber 1.A protective coating formed of an insulating material (dielectric) 102is disposed on the surface of the inner wall 101 of the reaction chambercontainer 10, which is formed by anodizing (providing alumite treatmentto) the surface of an aluminum alloy base material. The wafer 4 to beetched is mounted on an electrostatic chuck not shown formed of adielectric film disposed on the wafer holding electrode (substrateholder) 14, and the wafer 4 is chucked onto the holder by electrostaticforce. Helium gas is filled between the wafer 4 and the electrostaticchuck film so as to ensure thermal transmission between the wafer andelectrode 14 to thereby control the temperature of the wafer 4. Awafer-biasing high-frequency power supply 19 is connected to the waferholding electrode 14 so as to apply bias high frequency to the wafer 4.

In order to perform etching, reactive gas is introduced to the reactionchamber 1 through the gas discharge plate 13 while retaining a pressureranging typically between 0.5 Pa and 10 Pa, magnetic field is applied tothe reaction chamber 1 through the field coil 15, and output from theplasma-generating high-frequency power supply 18 is applied through theplasma-generating high-frequency electrode 17 disposed on the upper areaof the reaction chamber 1 into the reaction chamber 1, according towhich plasma 9 is generated in the reaction chamber 1.

The gaseous species used for generating plasma can be selected accordingto the object of the process, and for example, if a Si-based etchingprocess is to be performed, reactive gases such as Cl₂, HBr, CF₄ and SF₆and gases such as O₂ and Ar are used in combination. These reactivegases react chemically with the conductive material such as aluminumalloy and stainless steel constituting the reaction chamber container10, which may cause damage to the inner wall 101 of the chamber, and ifthe metallic compounds such as AlCl and AlF generated as chemicalreaction products from the wall material enter the plasma andcontaminate the circuit element formed on the wafer, the electricproperties of the circuit element is affected and the performancethereof is deteriorated.

In order to prevent such metal pollution caused by the wall material,according to the present invention, the surface of the aluminum alloy isanodized so as to protect the area coming into contact with plasma withan alumite or anodized aluminum (dielectric coating) that is chemicallystable. The area to which the protective coating is formed by anodizingaluminum must at least correspond to the area of the wall directlyexposed to high-density plasma. From the aspect of reducing metalpollution, however, it is effective to provide a protective coating notonly to a limited portion of the reaction chamber but to the whole innerwall surface including the exhaust port 16 to which diffused plasma canpossibly come into contact with.

The magnetic field applied to the reaction chamber 1 functions not onlyto effectively retain the electrons in the plasma that are acceleratedby the UHF-band high-frequency supplied from the high-frequency powersupply 18 so as to enhance the plasma generation efficiency, but also tomanipulate the conveyance of the generated plasma to the wafer locationby controlling the shapes of magnetic lines of force and to adjust theplasma distribution. Regarding density distribution of plasma 9 withinthe reaction chamber 1, the plasma density is highest at the upper areaof the reaction chamber 1 which is the plasma generating region, andalong with the diffusion of plasma from the generation region toward thewall of the reaction chamber 1, the plasma density is reduced. At thistime, if magnetic field is applied, the electrons and ions are noteasily transported in the direction traversing the magnetic lines offorce due to the Lorentz force operating thereto, so the plasmadistribution at the downstream area near the electrode 14 and the innerwall 101 of the reaction chamber depend greatly on the magnetic fieldconfiguration.

With reference to FIG. 2, the status of distribution of the plasmadensity within the reaction chamber 1 is explained in detail. Thecontour 91 of plasma 9 illustrates an outline of how the plasma densityis distributed when a magnetic field exists. The variation of plasmadensity is small along the magnetic lines of force 50 generated by thefield coil 15, and the variation of plasma density is great in thedirection traversing the magnetic lines of force 50. The potentialdistribution in the plasma corresponds substantially to this densitydistribution, wherein the potential difference is small along themagnetic lines of force, and the potential difference is great in thedirection traversing the magnetic lines of force. According to theexamples illustrated in FIGS. 1 and 2, the conductive members 21 a and21 d are disposed in the areas where the plasma density and plasmapotential are higher compared to those at the side wall of the innerwall 101 of the reaction chamber 1. That is, according to FIG. 1, theconductive member is disposed at a position where the potential issubstantially equal to the plasma potential at the side wall of theinner wall 101, and according to FIG. 2, the conductive member 21 d isdisposed to reach a position where the potential is higher than theplasma potential at the side wall of the inner wall 101. As for theshape and position of the conductive member 21, it is also possible todispose a single rectangular conductive member near the substrateholder. However, if the process requires a highly accurate symmetricproperty, it is effective to dispose plural conductive members 21 atpositions surrounding the substrate holder at equal distances or todispose a ring-shaped conductive member 21 surrounding the substrateholder. The conductive member 21 is disposed so that the conductivematerial thereof comes into direct contact with plasma so as to enabledirect current to flow therein from the plasma, and the conductivemember is either connected to the earthed reaction chamber container 10made of conductive metal or earthed through a wire connection so as toallow the incoming direct current to flow to the earth.

The DC earth is disposed where the plasma floating potential is higherthan the plasma floating potential at the inner wall 101 near thesubstrate holder (wafer holding electrode) where much of thehigh-frequency current applied to the substrate holder flows and causeschipping of walls. Thus, the floating potential of plasma near thesurface of the inner wall 101 is reduced to either ground potential ornegative potential by the DC earth, and sputtering by ions issuppressed. The area of the inner wall or earth through whichhigh-frequency current flows vary according to the plasma generationmethod, the shape of the reaction container 10 and the wall material, soit is necessary to optimize the location of the DC earth to correspondto the position where sputtering must be suppressed. Further, byapplying a magnetic field to the plasma, manipulating the fieldconfiguration thereof and controlling the plasma potential distribution(density distribution), the effect of the DC earth can be furtheroptimized.

The result of measuring using an oscilloscope the potential waveforms atthe side wall surface of the inner wall 101 of the reaction chamber 1having the structure illustrated in FIG. 1 are illustrated in FIG. 3 andFIG. 6. The measurement of the potential was performed on the side wallof the inner wall 101 at an intermediate height between the wafer 4 andthe gas discharge plate 13 separated with a distance of 100 mm. In thismeasurement, CF₄ gas was used as discharge gas, pressure was set to 2Pa, and discharge power was set to 500 W. The bias high frequencyapplied to the wafer 4 with a diameter of 300 mm had a frequency of 400kHz, and the power thereof was varied from 50 W to 160 W. If aconductive member 21 functioning as the DC earth was not provided, asshown in FIG. 6, the measured inner wall surface potential was variedfor approximately 5 V with the frequency of the bias high frequencyapplied to the wafer, and the direct current potential was changedgreatly along with the bias power. The level of variation of directcurrent potential in this case was changed in the range betweenapproximately −12 V and 14 V, and the potential variation wasapproximately 26 V. In general, the threshold energy of ion sputteringis approximately over 10 V, so a potential variation of approximately 26V increases ion sputtering.

FIG. 3 illustrates an example where the conductive member 21 a isdisposed inside the reaction chamber 1. In the measurement, theconductive member 21 a used experimentally was a stainless steel sheet(0.1 t) having an area size of 50 mm×60 mm, and was located at theposition illustrated in FIG. 1. The conductive member 21 a was retainedon the surface of the alumite 102 at the lower end of the side surfaceof the inner wall 101 of the reaction chamber 1 not shown in FIG. 1, andwas earthed through a cable. Thus, when the plasma is earthed throughthe conductive member 21 a disposed at a position illustrated in FIG. 1,the surface potential on the inner wall 101 of the reaction chamber 1varies for substantially 5 V at 400 kHz, but the variation of directcurrent potential caused by the bias high-frequency power as shown inFIG. 6 is suppressed, so that not only the variation is reduced greatly,but also the overall potential waveform is maintained at negativepotential. As a result, the ion-accelerating potential difference fromthe plasma can be reduced, by which the chipping of the walls issuppressed and the metal contamination caused by the wall material isprevented.

A protective coating made of insulative ceramic such as carbide, oxideor nitride (for example, SiC, boron carbide and alumite) that arechemically stable materials, are utilized as protective coating(dielectric coating) 102 on the inner wall 101 of the reaction chamber.The relationship between the coating thickness d, the frequency f of theplasma-generating high-frequency applied to the substrate 4 and thedielectric constant ∈ of the dielectric is determined so that theimpedance per unit area R=d/(2πf∈) when the above-mentioned highfrequency is propagated through the dielectric portion by capacitycoupling is 100 Ω or smaller. Thus, with respect to high frequency, theimpedance R of the protective coating can be reduced, and the propertyas the earth is not deteriorated. As a result, even when the inner wallof the reaction chamber is covered with protective coating, theimpedance of the protective coating against the high frequency currentflowing in through the walls or into the earth is increased, by whichthe increase of plasma potential variation can be suppressed and thechipping of walls by ion sputtering can be prevented.

The mixing in of metal to the plasma was evaluated by the aluminumcontent in the deposition adhered to the gas discharge plate 13 made ofquartz disposed at the upper portion of the reaction chamber. When thealuminum from the aluminum alloy material forming the wall is mixed intoplasma, it reacts with the fluorine-based gas used widely in etchingprocesses, forming a chemically stable AlF compound that tends to bedeposited inside the reaction chamber 1 as deposit and causescontamination. Therefore, with the aim to reduce the amount of Alcontained in the deposit, the areas of the conductive members 21constituting the DC earth were varied to test how the chipping of wallswas reduced thereby, the result of which is shown in FIG. 4. As shown inFIG. 4, the amount of Al contained in the deposit was reduced in inverseproportion to the area of the conductive member 21. Regarding the effectof reducing the amount of Al, the aluminum content was reduced byapproximately ⅓ when the DC earth area was 5 cm², while the content wasreduced by approximately ⅔ when the earth area was 33 cm², showing atendency to saturate. According to the present experiment, it wasdiscovered that the required DC earth area was approximately 5 cm² orgreater and 33 cm² or smaller. The area of the conductive memberdescribed here corresponds to approximately 0.3% to 2.5% of the wholesidewall area of the reaction chamber 1. Therefore, in general, chippingof the side walls can be effectively suppressed by providing aconductive member with an area corresponding to approximately 0.3% to2.5% of the sidewall area that functions as an effective earth for thebias high frequency. However, when considering the difference in effectsresulting between the various equipment configurations, such as betweena parallel plate plasma equipment and an induction RF plasma equipment,it is considered desirable to provide a conductive material having anarea of less than approximately 10% of the whole reaction chamber wallarea to function as the effective earth for high frequency to obtainadvantageous-results without fail. However, even if it is impossible toacquire a large DC earth area, it is still possible to expect an Alcontent cut-down effect of approximately 1/10 with a DC earth areacorresponding to 0.1% of the whole reaction chamber wall area.

According to some plasma processing conditions, etching reactionbyproducts or deposition caused by CVD processes are deposited on thesurface of the DC earth, inhibiting the flow of direct current to the DCearth, and sometimes even disabling the function of the earth for thedirect current when the amount of deposition is excessive.Conventionally, when performing an Si-based etching process, a mixed gasof Si and chlorine or HBr is used. By adding thereto a fluorine-basedgas (such as SF₆) having the effect to remove Si-based reactionbyproducts, the deposition of Si-based reaction byproducts to the DCearth surface can be suppressed, and the functions of the DC earth canbe maintained stably. It is also effective to perform a normal etchingprocess to one or more wafers before generating plasma using the mixedgas of fluorine-based gas or chlorine-based gas to remove the depositionon the DC earth surface in order to maintain the effects of the DCearth.

The embodiments concerning the structure of the conductive member 21 areshown in FIG. 5. Conductive ceramic or SiC and the like are preferableas the material for forming the conductive member 21 considering theresistance to reactive gas such as chlorine-based gas. One effectivemethod of mounting the conductive member 21 to the reaction chamber isillustrated in FIG. 5( a), in which a conductive member 33 having goodcontact property to both the reaction chamber container 10 and theconductive member 21 is deposited on the mounting surface by evaporationor the like in order to assure a good electric contact between theconductive member 21 and the surface of the earthed reaction chamberinner wall 101, and a screw 31 formed of conductive material is used tomount the conductive member 21 to the mounting surface.

Another example is shown in FIG. 5( b), in which a conductive member 21is formed by depositing SiC or conductive ceramic via CVD or thermalspraying to the surface of a metallic conductive member 32 coming intocontact with plasma, and fixing the conductive member 21 on the reactionchamber inner wall 101 with screws 31 formed of conductive material.According to this example, a protective coating against reactive gas isformed, and the earth can maintain its function stably for a long time.

Another example is shown in FIG. 5( c), in which a conductive protectivecoating 21 is formed to the portion of a metallic conductive materialscrew 31 coming into contact with plasma, and mounting the screw ontothe inner wall 101 of the reaction chamber, thereby providing a small,inexpensive conductive member 21 that can be mounted easily to thedesired location.

According to the mounting method illustrated in FIG. 5( a), it isnecessary that the area of the inner wall 101 on which the conductivemember 21 is mounted is not covered with insulating material since aconductive material 33 is deposited thereon, but according to themethods illustrated in FIGS. 5( b) and 5(c), the conductive member 21and the inner wall 100 of the reaction chamber are communicated via aconductive screw 31, so there is no problem even if the surface of theinner wall 100 of the reaction chamber is covered with insulatingmaterial.

As described above, when the base material of the DC earth is not metal,such as conductive ceramic, SiC, Al and Al compound, the earthresistance of the DC earth can be reduced by interposing on the mountingsurface of the DC earth a conductive material with a dielectric constantof 1 Ωcm or less via evaporation, spraying, sandwiching or the like.Furthermore, the present invention can be applied not only to the plasmaprocessing apparatuses as described above, but also to processingapparatuses that generates plasma within a reaction chamber by applyinghigh frequency and then applies a second high frequency to a substrateholder on which a substrate is mounted to control the ion energy to thesubstrate and to provide plasma processing to the substrate, theapparatus having its inner wall surface covered with insulating materialso as to prevent the inner wall of the chamber from being sputtered byplasma and contaminating the processed substrate.

1. A plasma processing apparatus for processing a substrate with plasmaby applying a high frequency to a reaction chamber so as to generateplasma therein, and applying a second high frequency to a substrateholder on which the substrate is placed so as to control the ion energyto the substrate; wherein a dielectric that is exposed to the plasmasubstantially covers a surface portion of an inner wall of the reactionchamber, an electrically conductive member is disposed within thereaction chamber so as to be exposed to the plasma within the reactionchamber at a position with respect to the inner wall of the reactionchamber which is covered with the dielectric, and the electricallyconductive member is electrically coupled to earth one of directly andthrough the inner wall of the reaction chamber so as to form a DC earthwhich enables direct current to flow therein from the plasma, theelectrically conductive member has an area in a range of 0.1% to 10% ofthe inner wall area of the reaction chamber, a magnetic field generationmeans is disposed outside of the reaction chamber so as to apply amagnetic field to the plasma, and the electrically conductive memberforming the DC earth is disposed at a position crossing a magnetic lineof force that is closer to the substrate holder than a magnetic line offorce that crosses the inner wall of the reaction chamber having thedielectric thereon.
 2. The plasma processing apparatus according toclaim 1, wherein the dielectric covers 90% or more of a total surfacearea of the inner wall of the reaction chamber.
 3. The plasma processingapparatus according to any one of claims 1 and 2, wherein theelectrically conductive member forming the DC earth is located at aposition within the reaction chamber where a floating potential ofplasma is substantially equal to or greater than a floating potential ofthe plasma at the inner wall of the reaction chamber covered with thedielectric with respect to the high frequency or the second highfrequency.
 4. The plasma processing apparatus according to any one ofclaims 1 and 2, wherein the dielectric is a protective coating formed ofinsulating ceramic such as carbide, oxide or nitride, as exemplified bySiC, boron carbide and alumite, and a thickness d of the dielectriccoating is determined so that, with respect to the relationship betweenfrequency f of the high frequency applied to the substrate and thedielectric constant ∈ of the dielectric, an impedance per unit area R=d/(2πf∈) when high frequency is propagated by capacitive couplingthrough the dielectric is 100Ω or smaller.
 5. The plasma processingapparatus according to any one of claims 1 and 2, wherein either a basematerial of the electrically conductive member forming the DC earth or aprotective coating disposed on a surface of the electrically conductivemember forming the DC earth and coming into contact with the plasma iscomposed of conductive ceramic, SiC, Al or Al compound.
 6. The plasmaprocessing apparatus according to any one of claims 1 and 2, whereinwhen a base material of the electrically conductive member forming theDC earth is composed of a non-metallic material such as conductiveceramic, SiC, Al or Al compound, a conductive part having a conductivityσ of 1 Ωcm or less is provided to a surface of the base material byevaporation, spraying or interposing, thereby reducing an earthresistance of the electrically conductive member forming the DC earth.7. The plasma processing apparatus according to claim 3, wherein thedielectric is a protective coating formed of insulating ceramic such ascarbide, oxide or nitride, as exemplified by SiC, boron carbide andalumite, and a thickness d of the dielectric coating is determined sothat, with respect to the relationship between frequency f of the highfrequency applied to the substrate and the dielectric constant s of thedielectric, an impedance per unit area R=d/(2πf∈) when high frequency ispropagated by capacitive coupling through the dielectric is 100Ω orsmaller.
 8. The plasma processing apparatus according claim 3, whereineither a base material of the electrically conductive member forming theDC earth or a protective coating disposed on a surface of electricallyconductive member forming the DC earth coming into contact with theplasma is composed of conductive ceramic, SiC, Al or Al compound.
 9. Theplasma processing apparatus according to claim 3, wherein when a basematerial of the electrically conductive member forming the DC earth iscomposed of a non-metallic material such as conductive ceramic, SiC, Alor Al compound, a conductive part having a conductivity σ of 1 Ωcm orless is provided to a surface of the base material by evaporation,spraying or interposing, thereby reducing an earth resistance of theelectrically conductive member forming the DC earth.
 10. The plasmaprocessing apparatus according to claim 1, wherein the electricallyconductive member is disposed within the reaction chamber and iselectrically coupled to earth by a wire extending through the inner wallof the reaction chamber.
 11. The plasma processing apparatus accordingto claim 1, wherein the electrically conductive member is positioned inthe reaction chamber so as to enable suppression of chipping of thesurface portion of the inner wall of the reaction chamber.